1. Field of the Invention
The present invention generally relates to a method for transferring wafer cassettes between semiconductor device process equipment and an Automated Guide Vehicle (AGV) at a semiconductor fabrication facility. More specifically, the present invention relates to a method that includes steps for checking the mode of the process equipment and changing the mode, if necessary, to a mode more suitable for the transfer that is to take place.
2. Description of the Related Art
The processes employed during the fabrication of semiconductor devices have become increasingly automated. Until recently, however, the transfer of semiconductor wafer cassettes among pieces of processing equipment has been a manual process involving a human operator. With the increasing integration of semiconductor devices, particles generated by the human body, formerly not considered to be a problem, have become a main cause of contamination. Consequently, much research and development has gone into automating the transfer of wafer cassettes. Automated Guide Vehicles (AGVs) have been developed to deliver cassettes under the remote control of a computer. Although great advances have been made, current systems still suffer some disadvantages. For example, if the process equipment is not in the correct mode when the cassette delivery unit arrives, the transfer may be initiated nonetheless. The result is that cassettes may not be transferred at all, or, worse, cassettes can be dropped and wafers spilled and broken.
FIG. 1 is a schematic diagram of a current wafer transfer system using communications links between an AGV and its controller and between the AGV and some semiconductor fabrication process equipment. The system includes transfer equipment 10, process equipment 20, and a host computer 30. The transfer equipment 10 includes a controller 11 that itself is connected to and controlled by the host computer 30. The transfer equipment 10 also includes an AGV 13 that operates under radio control by the controller 11 through a radio link represented by the arrow 12. The process equipment 20 is simultaneously controlled by the host computer 30 through a second controller (not shown). The host computer 30 communicates with the transfer controller 11 and the process equipment controller (not shown) according to a Semiconductor Equipment Communication Standard (SECS). The AGV 13 communicates with the process equipment 20 through a communications link represented by another arrow 15. In the current system, a photo-coupled Parallel Input/Output (PPIO) link serves as the communications link 15.
A cassette transfer occurs when a wafer cassette (not shown) is loaded into the process equipment 20 from the AGV 13 or when a wafer cassette is unloaded from the process equipment 20 to the AGV 13. The process equipment 20 includes a loading/unloading cassette stage 21 (FIG. 2) used when wafer cassettes are transferred between the process equipment 20 and the AGV 13.
FIG. 2 is a schematic diagram showing the loading/unloading cassette stage 21 of the process equipment 20. The stage 21 contains a sensor S1 that can detect the presence of a cassette when the cassette is placed in a preferred location A on the stage 21.
The operation of the current wafer cassette transfer system will be described with reference to FIG. 1 and FIG. 2. Starting with FIG. 1, a command to transfer a wafer cassette is issued by the host computer 30 which communicates the command through the connections to the transfer equipment controller 11 and to the controller (not shown) of the process equipment 20 according to SECS. The transfer equipment controller 11 then utilizes the radio link 12 to instruct the AGV 13 to move toward the process equipment 20. As a result the AGV 13 arrives at the process equipment 20 adjacent to its loading/unloading cassette stage 21.
After arrival at the process equipment 20, the AGV 13 begins to communicate with the process equipment 20 via the communications link 15. In the case of the PPIO link, the communications protocol is defined by the Semiconductor Equipment and Material International (SEMI) standard. The process equipment 20 has internal mechanisms (not shown) that must be deployed in a certain fashion for loading or unloading operations. For example, the process equipment 20 may have a door that must be opened, or the process equipment 20 may have a robot arm that must be unfolded during loading and unloading. In addition, the cassette stage 21 must be occupied by a cassette to allow unloading to occur, or the stage 21 must be empty to allow loading to occur. The state of such various internal mechanisms and the state of occupancy of the stage 21 constitute the transfer mode of the process equipment 20. In the conventional wafer transfer system of FIG. 1, the process equipment 20 is considered to be in a mode suitable for the transfer as commanded by the host computer 30 when the AGV 13 arrives and begins communicating, regardless of whether the wafer transfer system is actually ready to load or unload a cassette.
For various reasons, however, the process equipment 20 may not be in a suitable transfer mode when the AGV begins communicating. When transfer begins while the processing equipment 20 is not in a suitable mode, for example, if the door is not completely open, or the robot arm is not completely unfolded, the cassette might not be placed on the stage 21 at the preferred location A in FIG. 2.
FIG. 2 shows that when a cassette is placed at the preferred location A on the stage 21, the sensor S1 can detect its presence. However, if the cassette is placed near the preferred location but not in the view of the sensor S1, the sensor S1 may indicate to the process equipment controller that the stage is empty. In this case, the process equipment controller will not clear the stage upon receiving the load command from the host computer 30. When the AGV 13 attempts to perform the load operation, the cassettes collide and the transfer operation fails. This faulty detection of cassette stage 21 occupancy is a main cause of failure for the transferring operations. In addition, cassettes may collide with a door that is not open or a robot arm that is not unfolded. For any of these reasons a cassette may be dropped during the failure of the transfer operation and wafers may be spilled. The current system is unable to check whether there are problems with the stage 21 occupancy or the internal mechanisms of the process equipment 20 prior to transferring wafer cassettes.
A need exists, therefore, for a wafer cassette transfer system that will not transfer cassettes if the process equipment is not in a suitable mode.